1. Field of the Invention
The present invention is related generally to preparation of thin film compounds and more particularly to preparing thin film compounds of Cu(In,Ga)(Se,S).sub.2 in semiconductor devices.
2. Description of the Prior Art
Thin films of copper-indium-diselenide (CuInSe.sub.2), copper-gallium-diselenide (CuGaSe.sub.2), and copper-indium-gallium-diselenide (CuIn.sub.1-x Ga.sub.x Se.sub.2), all of which are sometimes generically referred to as Cu(In,Ga)Se.sub.2, have become the subject of considerable interest and study for semiconductor devices in recent years. Sulfur can also be, and sometimes is, substituted for selenium, so the compound is sometimes also referred to even more genetically as Cu(In,Ga)(Se,S).sub.2 to comprise all those possible combinations. They are of particular interest for photovoltaic device or solar cell absorber applications because of solar energy to electrical energy conversion efficiencies that have, prior to this invention, been shown to exceed fifteen percent (15%) in active areas and to approach fourteen percent (14%) in total areas, which is quite high for current state-of-the-art solar cell technologies, where the theoretical efficiency limit for this type of thin film solar cell is about 23% to 25%. Prior to this invention, which has demonstrated a new world record for light to electrical energy conversion efficiency of 16.4%, the previous world record was 14.9% set by a European University consortium in January 1993.
It has been generally believed by persons skilled in this art that the best electronic device properties, thus the best conversion efficiencies, are obtained when the mole percent of copper is about equal to the mole percent of the indium, the gallium, or the combination of the indium and gallium in the Cu(In,Ga)Se.sub.2 compound or alloy. The selenium content will not generally be important to the electronic properties of the semiconductor if the growth conditions supply sufficient selenium so that it comprises about fifty atomic percent (50 at. %) of the Cu(In,Ga)(Se,S).sub.2 compound to form the desired crystal lattice structures.
While growth of single crystal CuInSe.sub.2 has been studied, such as in the U.S. Pat. No. 4,652,332, issued to T. Ciszek, the use of polycrystalline thin films is really more practical. Sputter depositing a ternary single phase CuInSe.sub.2 layer, including the ability to determine the properties of the thin film, such as multilayer structures, by varying the sputter process parameters, is described by the U.S. Pat. No. 4,818,357, issued to Case et al. However, the two fabrication methods of choice are: (1) Physical vapor deposition of the constituent elements, exemplified by the process disclosed in the U.S. Pat. No. 5,141,564, issued to Chen et al., is generally used as a research tool; and (2) The selenization of Cu/In metal precursors by either H.sub.2 Se gas or Se vapor. The selenization technology generally exemplified by the processes described in the U.S. Pat. No. 4,798,660, issued to Ermer et al., the U.S. Pat. No. 4,915,745, issued to Pollock et al., and the U.S. Pat. No. 5,045,409, issued to Eberspacher et al., is currently favored for manufacturing processes. However, thin films produced by the selenization processes usually suffer from macroscopic spatial nonuniformities that degrade performance and yield, and reproducible consistent quality from run to run is difficult to obtain and unpredictable. Therefore, working with Cu(In,Ga)(Se,S).sub.2 material has still been difficult, particularly when scaling up, so it has yet to be commercialized.